For controlling flow in conducts in many fields of application, particularly in hydraulic and pneumatic installations, as in the case of hydraulic, heating, irrigation, compressed air distribution and so on, valves with special characteristics are widely used. Such valves, among other things, must provide a high degree of resistance to the fluid even in the presence of high pressures, they must be easily nd quickly maneuverable, offer limited head loss in the "open" position and, furthermore, be of low cost.
For this purpose so-called "ball" valves are widely used. These valves, however, even though offering a good performance, are relatively expensive, due to the number of parts they are made of and the assembling operations thereof, and require, especially when of larger dimensions, a great deal of stress when the valve is manipulated.
So-called "butterfly" valves are also known. These valves feature a substantially disc-shaped shutter, which can be rotated between a position transversal to the flow, where it effects blocking, and a position parallel to the flow in which passage of the fluid is allowed.
To form the seals in these valves, contact metal surfaces can be provided which, however, do not guarantee a full sealing. Valves where a gasket of elastomeric material is set in the perimeter of the disc and seals against the inner surface of the valve body are also known; these valves, however, display problems concerning the shutter closure, after a certain period of inactivity, due to the fact that the gasket stressed seating of the shutter contrasting its deformation in one differently when the shutter is closed and when the shutter is open.
This state of stress gives rise to, with a deformation wide enough to realize the sealing, to values near to or exceeding the limit of elastic deformability of the material and under such conditions permanent modifications of the seal material can occur which can affect the seal and even bring about adhesion.
Such valves, furthermore, tend to be costly, as they require mechanical work for both seating of the gasket on the shutter and stressing of the sealing surface of the gasket against the valve body, and also display, due to the space longitudinally occupied by the gasket, high flow head losses. Valves of the butterfly type are also known, whith a sealing element made of a ring of elastomeric material housed in the external body, which however presents sealing problems between the outer, smooth surface of the external body, and the valve body, because the seal is realized on a very wide surface and with limited radial loads. In filtrations of fluid between the sealing element and the valve body can moreover, give rise to an inward buckling of the sealing element, which has a low rigidity because of its elastic deformability which must be sufficient to ensure proper sealing. Therefore these valves are found to be suitable only for use at limited pressure values. Beside, these valves, due to the characteristics of the sealing material, are used only with water.